Hi, All. I just purchased a Miracle 20 and what I've been reading about tuning the rig and rotating the mast leaves me a little confused. The Hobie and H20 specific tuning guides recommend spreader rake, increasing tension on the diamond wires as the wind comes up and decreasing rotation of the mast as the wind increases. Phil Berman and other Catamaran books are calling for just the opposite: Less diamond wire tension to allow mast bend to flatten the sail and more mast rotation to flatten the sail out. The only reference I've been able to find that accounts for this radical difference is Sideways Bend vs. PreBend. It that the issue here? I'd love to hear some long winded and technical discussion of what is actually happening with the rig and sails under varying wind and tuning conditions. I prefer to understand what is happening rather than just follow instructions.
TC

I'd suggest you pick up Rick Whites book, catamaran racing for the 90s'. I think Berman is a little outdated when it comes to pre-bend, etc. Rick explains the old thinking and new techniques very well. Many of the techniques of the early 80s' have been disproven. With the creation of square top sails it seems everything points to pre-bend and rotation rather than diamond tension. Also there is a specific section on the miracle.

With tight diamond wires and spreader rake, you are prebending the mast. This flattens the sail by pulling shape out of the luff curve of the sail. A straight mast will give you more initial shape because you are pushing the round luff curve straight. You can not pull all of the shape out of the sail though, because most sails also have shape sewn in at broad seams. Downhaul flattens the sail as it both bends the mast more and streaches the sail cloth on the bias opening the head of the sail. If you rotate more with tight diamond wires, you are pulling against the stiffness caused by the diamond wires as you sheet in. This results in less additional bend and more power. Less rotation and you will gain more bend, flattening the sail even though you are pulling along the longer axis of the mast. Additionally, the less rotated mast has less drag.

Both loose and tight diamonds are just ways to control/limit mast bend. Tight diamonds just allow you to pre-bend the mast and limit mast area drag.

Thanks for the good responses, particularly Hammond, who explained what is going on with the mast and sail. As someone who's been racing monohulls w/ non-rotating masts, The cat has been a little hard to wrap my head around. I will definitely look up Rick White's book and I certainly do trust what the Hobie guides are saying. I just want to understand it a little better. Thanks again.

the best way to depower the H20 on the water is to reduce rotation of the mast. you can additionally close the slot between the jib and main.
If you have a furling jib you can always furl it first.

on shore you can tighten diamond wire tension.

if the boat is not assembled for sailing you can increase spreader rake to help depower the boat. However I have found once you get the boat handling the way that works the best for your crew weight and experience then leave the spreader rake alone and use the other adjustments.

the best method is to take a tune up lesson from an expereinced H20 driver and have him walk you thru the various ways of setting up your boat to acheive the most preformance from it for your skill level. All the skippers I know are more than willing to assist a newer one.

You don't say where you are located but there is probably a local Hobie Fleet or sailor that can help. Let me know where you are and I will try to help you find one.

Thanks again for all the excellent responses. I made my way through Rick White's "Catamaran Racing for the 90s" and he explains it quite well. In brief, for anyone who might be following this thread, the difference lies in the sidebend method of sail shaping popular in the 80s and the pre-bend method that evolved in the 90s. The side bend method relies on slack diamond wires and mast rotation to flatten a full sail. This, however, allows the mast to bend into slot between the main and jib, disrupting the airflow. The only way to counter that is to over-rotate the mast, which creates a poor entry profile for the wind on the main. Pre-bend, with its tight diamond wires, keeps the mast straighter and relies on downhaul and outhaul primarily to flatten the sail. This keeps the slot open and rotation is primarily used to create a smooth entry profile for the main. There are some other factors at work, of course, and any errors I have made here are purely my own, but now I feel I understand the why of the tuning adjustments and not just the how. Thank you all again for your responses.
Todd Craig